This invention relates generally to the field of optical displays, and more particularly pertains to calibrating tiled displays using multiple projectors to produce a large, and/or a higher resolution image.
Multiple projection displays have been proposed and used for many years. In the 950""s, the xe2x80x9cCINERAMAxe2x80x9d system was developed for the film industry. The CINERAMA system used three films to project three images using three separate projectors, which were then combined to form a single panoramic image. Disneyland continues to use a similar multiple projector system, wherein a circle of projectors shine onto a screen that circles the wall of a round room.
In the video field, multiple projector systems have been proposed and used for a number of specialty applications. U.S. Pat. No. 4,103,435 to Herndon and U.S. Pat. No. 3,833,764 to Taylor suggest using multiple projector systems for flight simulators. In many of these systems, multiple video screens are placed next to each other to form a large tiled image display.
A difficulty with many of the video based multiple projector display systems is that the multiple images often do not appear as one single continuous image on the display screen. When multiple images are projected side-by-side and/or top-to-bottom on a single screen, there is normally a seam or overlapping region between the images for example, an Mxc3x97N projector array, where M and N are generally expressed as positive integer values, though, in using overlap and portions of displays, fractional values may be assigned. Also Mxc3x97N projector arrays can be arranged to have constant and identical overlap or can be arranged to have varying degrees of overlap, depending on one""s optimization criteria, which can include reliability, fault tolerance, cost and performance. The final display image will either appear as multiple images placed side-by-side with a gap between images or, if the images are made to overlap on a single screen, with a bright line or band there between. In the region of overlap the light from each projector will add to the output light of the other. This applies to the black output level as well. Ideally, when displaying a black image, this region of overlap should be generally uniformly black across the entire displayed image. Instead one generally observes the black image to brighten in the regions of overlap. When the images of two projectors overlap, the amount of light in the overlapped regions of the images is approximately double the amount of light observed on the screen in regions where only a single projector image resides; in regions where four projected images overlap, the amount of light is approximately four times that of the single projector image, and so on. Thus, the observer of the screen will generally see the output image containing objectionable artifacts. The same effects happen for white images, and for all images in between black and white. Generally, the black and the white images may be conceptualized as the upper and lower reference levels for constructing any image whose content spans these two extremes.
The prior art, for example, in the Panoram tiled display resolved overlap issues by requiring the display device to have a black reference level having very low stray light. This needed CRTs, because CRTs have huge native contrast ratios and deep black, several times darker than other common display media such as LCD projectors and DMD projectors. While the deep-dark display or CRT-only architecture might work well for many applications, it fails to meet requirements found in the cinematic, medical and other industries demanding high image quality and performance. In these high performance applications, the contrast ratio requirements often exceed 1000:1. The cinema industry generally requires 1500:1, and the medical industry generally requires displays for digital radiography having contrast ratios in the range of about 2000:1 to 4000:1. With the contrast ratio of CRTs at that or a lesser range, any overlapping strategy as used in the CRT-only architecture fails. It divides the contrast ratio by the number of CRTs used. Thus, for a cinematic application requiring a contrast ratio of 1500:1, any overlap of the CRTs will shrink the contrast ratio to 750:1 in the region of overlap. Any regions having four CRTs overlapping in a 2xc3x972 matrix, will show a quadrupling in brightness and thus a reduction in contrast ratio to a mere 375:1. This is observable and generally objectionable in the industry.
Attempts have been made to hide such artifacts, one such example being raising the regions of non-overlap to the same brightness levels as the regions of overlap. Such practices are usually implemented by adjusting the input video level to obliterate the visibility of the regions of overlap. However, this method reduces the contrast ratio over the entire display, even in areas where only a single projector projects its image content. And in cases where multiple CRTs or other imaging devices overlap their imagery, the contrast ratio over the entire display will be compromised accordingly.
What is desired is a fall-off in intensity of black and white levels such that the superposed images produce a uniform luminance from center to edges, including the overlapped regions of the projected image. In practice, such an ideal luminance profile is difficult to achieve, as the displays generally exhibit a fall-off in intensity from the center of a displayed image to its edges. To attain a uniform, center to edge luminance profile requires clipping the display""s native intensity at display center and elsewhere to the same value as at the edges. Unfortunately, this will result in the loss of the display""s native brightness and will significantly reduce the power to image brightness conversion efficiency of the system. Another method based on insensitivity of human vision to low spatial frequency changes is to allow a fall-off of the luminance profile near the edges of the tiled image. In theory, such profiles can be achieved electronically by adjusting the video going into each display. The corrective functions can be multiplied by the image content and will result in a uniform output over much of the gray scales to be rendered. However, the closer the input image approaches the black state, the more the actual deviates from the ideal using this method. This is because the input video commanding a black state on the display does not achieve a true black state in practice. This, as was explained above, is because display technologies generally pass or emit light even when displaying black.
To overcome the limitations described above, a pending commonly assigned patent application to Chen et al, suggest using an optical function generator to generate a spatial gradient profile, then to apply the spatial gradient profile to a spatial filter and then to dispose the spatial filter anywhere in an image formation path of each of the displays to produce a seamless tiled display image.
Generally, tiled displays require periodic re-calibration because the performance of their projectors and/or other hardware tend to change over time. To overcome this problem commonly assigned pending patent applications to Chen et al, suggest a method and apparatus to calibrate tiled displays not including spatial filters in their projectors. However, the method and apparatus disclosed in the pending patent application does not lend itself to calibrate the tiled displays including spatial filters in their projectors because the spatial filters generally change the luminance characteristics around the edges of the display. This can affect the displaying of a template used in the calibration process. Therefore, there is a need in the art for a method and apparatus to calibrate tiled displays including spatial filters in their projectors.
Also, the pending patent applications disclose a method and apparatus that requires a lot of memory and hardware to calibrate the tiled displays. Therefore, there is also a need in the art for a method and apparatus that requires less memory, less hardware, and has an improved processing speed to produce a high-resolution tiled image.
According to one aspect of the present invention, a method of calibrating a seamless tiled display image having multiple overlapping discrete images produced by multiple displays generates a display-to-screen spatial transformation function to reduce one or more undesirable geometric projector characteristics, for each of the projectors used in the tiled display. The method generates a screen-to-camera spatial transformation function to reduce one or more undesirable geometric camera characteristics for each of the cameras used in capturing the displayed images. The method requires generating a spatial luminance transformation function for effective color calibration for each of the display images in the tiled display. The method further requires inputting a high-resolution image into a tiled display processor to form the tiled images of the tiled display, and segmenting the inputted high-resolution image to form tiled images based on an array of images used in the tiled display. The method requires pre-warping the segmented tiled images using the display-to-screen spatial transformation function to reduce the one or more undesirable geometric projector characteristics. The method requires applying an inverse of the spatial-luminance transformation function to each of the pre-warped images to effectively blend colors in the tiled display images.
Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.